Specifications

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Chapter 15 Video Hardware
Dot Pitch (CRTs)
Another important specification that denotes the quality of a given CRT monitor is its dot pitch,
which is controlled by the design of the shadow mask or aperture grille inside the CRT. A shadow mask
is a metal plate built into the front area of the CRT, next to the phosphor layers. It has thousands of
holes that are used to help focus the beam from each electron gun so that it illuminates only one cor-
rectly colored phosphor dot at a time. Because of the immense speed of screen rewriting (60–85 times
per second), all dots appear to be illuminated at the same time. The mask prevents the electron gun
from illuminating the wrong dots.
In a monochrome monitor, the picture element is a screen phosphor, but in a color monitor, the picture
element is a phosphor triad—which is a group of three phosphor dots (red, green, and blue). Dot pitch,
which applies only to color monitors, is the distance (in millimeters) between phosphor triads, measured
from the center of a phosphor dot in a given triad to the same color phosphor dot in the next triad.
Screens with a small dot pitch have a smaller space between the phosphor triads. As a result, the picture
elements are closer together, producing a sharper picture onscreen. Conversely, screens with a large dot
pitch tend to produce images that are less clear. Figure 15.7 illustrates dot pitch.
GBRGBR
RGBR
GBRG
B
B
RGB
B
RG
B
RGB
B
RG
BRGB BRG
BR
GBRGBR
GBRGBR
G
B
Figure 15.7 Dot pitch is the distance between each group (triad) of red, green, and blue (RGB) phosphors.
A smaller dot pitch helps produce sharper, clearer images.
Note
Dot pitch is not an issue with LCD portable or desktop display panels because of their designs, which use transistors rather
than phosphor triads.
The original IBM PC color monitor had a dot pitch of .43mm, which is considered to be poor by
almost any standard. Smaller pitch values indicate sharper images. Most recent monitors have a dot
pitch between .25mm and .30mm, with state-of-the-art monitors down to .24mm or less. To avoid
grainy images, look for a dot pitch of .26mm or smaller. Be wary of monitors with anything larger
than a .28mm dot pitch; they lack clarity for fine text and graphics. Although you can save money by
buying monitors with smaller tubes or a higher dot pitch, the trade-off isn’t worth it.
Monitors based on Sony’s Trinitron picture tubes and Mitsubishi’s DiamondTron picture tubes use an
aperture grille, which uses vertical stripes (rather than a shadow mask) to separate red, green, and blue
phosphors. This produces a brighter picture, although the stabilizing wires shown in Figure 15.8 are
visible on close examination. Monitors using an aperture grille–type picture tube use a stripe pitch
measurement instead of dot pitch. An aperture grille monitor stripe pitch of .25mm is comparable to a
.27mm dot pitch on a conventional monitor.
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